The invention relates to a Wien-Robinson oscillator having an operational amplifier, which is provided with a negative feedback path and with a positive feedback path.
A Wien-Robinson oscillator such as this is known in particular from the text book by Tietze-Schenk “Halbleiter-Schaltungstechnik” [Semiconductor circuit technology], Springer Verlag, 8th revised edition, 1986, pages 454 to 458. The frequency of a Wien-Robinson oscillator is governed by RC elements, that is to say networks formed from resistors and capacitors. Oscillators such as these are preferably used in the audio frequency range, since in this frequency range, the inductances and capacitances of LC oscillators are inconveniently large. In the case of a Wien-Robinson oscillator, a Wien-Robinson bridge circuit, such as that illustrated in FIG. 15.26 on page 455, is used as a feedback network and as an element that governs the frequency. The fundamental circuit of an oscillator such as this is illustrated in FIG. 15.27 on page 456. A negative feedback path and a positive feedback path are arranged between the output of the operational amplifier and a reference ground potential. The negative feedback path essentially comprises two resistors, whose common circuit point is connected to the inverting input of the operational amplifier. The positive feedback path comprises two RC elements which are connected in series and whose common circuit point is connected to the non-inverting input of the operational amplifier. The RC element which is arranged between the non-inverting input of the operational amplifier and the reference ground potential comprises a resistor and a capacitor connected in parallel. The other RC element, which is arranged between the non-inverting input and the output of the operational amplifier, comprises a further resistor and a further capacitor connected in series. In order to satisfy the oscillation condition of the Wien-Robinson oscillator, the resistor which is arranged in the negative feedback path between the inverting input of the operational amplifier and the reference ground potential has half the resistance of the resistor which is arranged between the inverting input and the output of the operational amplifier. Furthermore, the resistors and the capacitors in the two RC elements of the positive feedback path are each of the same magnitude. In order to stabilize the output voltage of the oscillator, the resistor which is arranged between the reference ground potential and the inverting input of the operational amplifier in FIG. 15.27 is in the form of a fixed resistor and a controllable resistor connected in series. The controllable resistor is formed by the drain/source path in a generally symmetrical field-effect transistor, whose drain connection is connected to the reference ground potential. In the case of a symmetrical field-effect transistor, the source connection may alternatively be connected to the reference ground potential, instead of the drain connection. The controllable resistor in the form of a field-effect transistor allows the resistance ratio in the negative feedback path to be varied so as to satisfy the condition for the output voltage from the oscillator to have a constant amplitude. For this purpose, the output voltage from the oscillator is rectified, and the rectified voltage is supplied via a matching circuit to the gate connection of the field-effect transistor, as a control voltage. The amplitude level of the output voltage from the oscillator in this circuit depends on the data of the field-effect transistor. The stability of the output voltage from the oscillator can be improved—as is illustrated in FIG. 15.28 on page 458—by intermediate amplification of the gate voltage. Although the stated measures improve the stability of the output voltage from the oscillator, on the other hand, however, they also increase the circuitry complexity that is required for the oscillator. A further disadvantage of the described circuit arrangement is that the amplitude control system is highly sensitive to EMC interference. As normal, the letters “EMC” stand for “electromagnetic compatibility” in this case.